This invention relates to semiconductor device manufacture and, more particularly, to a method for supporting lead wire sets during the pellet mount and encapsulation steps of plastic encapsulated semiconductor device manufacture.
Semiconductor devices generally require environmental protection. To insure reliable operation, protection must be provided against the effects of, for example, mechanical abuse, water and water vapor, solvents, oils, greases and acids. Of course, it is rare that a single semiconductor device is exposed to, and thus requires protection from, all of the aforementioned hazards. Nevertheless, it is economically desirable to provide protection against most or all common potential hazards while utilizing only a single packaging approach. In this way mass production is facilitated and costs are reduced.
Plastic encapsulation has been found to be an effective way of physically and electrically protecting semiconductor devices at a relatively low cost. Plastic encapsulation has the further advantage of providing physical support for the several component parts forming the semiconductor device thus allowing the header assembly, which conventionally provides mechanical support, to be eliminated. However, the potential advantages of plastic encapsulation have not heretofore been fully realized as consideration of the following examples will indicate.
One method of manufacturing plastic encapsulated semiconductor devices begins with the step of punching a strip of copper to partially define leads. Thus, in the manufacture of transistors the copper strip is punched so that portions of three leads are formed. A sufficient portion of the copper strip is left intact to maintain the integrity of the strip and subsequent manufacturing processes are preferably performed in a batch process or mass production manner. After semiconductor pellets are affixed to the sets of three leads and electrical connections are provided, the portion of each device around the pellet is encapsulated. Typically, the portion of the copper farthest from the pellets supports the leads during the aforementioned steps. Following encapsulation that portion is removed thus separating the individual transistors and the leads from the strip. Often the leads thus formed are subjected to a rounding operation to remove the corners that are formed by the punching of the copper strip. The disadvantages of this system include the high number of steps required and the substantial amount of material wasted. Waste occurs because only a small portion of the original copper strip ultimately serves as part of finished devices. The remaining portion of the strip is waste material.
Another method of manufacturing plastic encapsulated transistors utilizes sets of lead wires that pass through small plastic beads and are thus coupled together. During fabrication, the sets of wires are individually handled as pellets and flying leads are affixed thereto. Ultimately, several sets of wires are placed in a rack that cooperates with a mold to facilitate plastic encapsulation. Following the encapsulation step, the plastic beads must be removed from the leads and discarded. The primary disadvantages of the system outlined immediately above include the waste of the the bead material which at least partially offsets the cost saving realized from the elimination of the header. Furthermore, handling is complex and requires many steps inasmuch as each bead and set of wires is handled individually for most of the fabrication operations.
It is, therefore, an object of this invention to provide a method for manufacturing plastic encapsulated semiconductor devices at a low cost, with a minimum number of steps and with a minimum waste of material.